1. Field of the Invention
This invention relates generally to electrical systems having a source of electrical power and an electrical load and more particularly relates to a safety switch for interrupting power to such a load in response to an excessive load current in order to prevent hazardous or damaging operation.
2. Description of the Related Art
There are many types of machines that transport people or move mechanical apparatus in the vicinity of people or otherwise require reliable control so they do not malfunction and cause personal injury or property damage. One of the most common electrical loads associated with such machines is an electric motor that is or drives a prime mover to move the mechanical apparatus. Such machines should not only operate when they are signaled or otherwise commanded to operate, but of more critical importance to safety is that they stop operating when they are signaled or otherwise commanded to stop. Although the invention is applicable to a broad variety of machines with electrical loads that have such control and safety requirements, it is illustrated in connection with one such machine, a wheelchair lift having an electric motor driven hydraulic pump as its prime mover.
In wheelchair lift systems, safety is probably the single most important factor. These lifts transport people who have a physical disability and it is particularly desirable to avoid jeopardizing them with apparatus that has the possibility of failing and causing personal injury.
FIG. 1 shows a prior art electrically operated hydraulic lift system. In such a system, electrical power is typically supplied directly from a 12 volt battery 1 through an overcurrent protection circuit breaker 2 to an electric motor M driving a hydraulic pump, which together are referred to as a hydraulic pump 4, and to internal electronics and accessories 7 that typically include flood lights. The size or capacity of the circuit breaker 2 is determined by the maximum load current anticipated from the sum of the hydraulic pump 4 current plus any additional loads that are powered such as the internal control electronics of the hydraulic lift system and any auxiliary loads 7 such as floodlights. A load power switch 3 is used to control power to the hydraulic pump 4. Because of the high amperage requirements of the hydraulic pump 4, the load power switch 3 will typically consist of a power contactor having a control coil, an input to which operates as a control input to the power contactor. Typically, the load power switch 3 is controlled by an operator from a manual switch 5 wired in series with a limit switch 6. The limit switch 6 detects a position of a mechanical component of the lift system.
In normal operation, assuming that the limit switch 6 is closed, closing the manual switch 5 commands and causes the load power switch 3 to close, in turn supplying power to the hydraulic pump 4 through the power switching terminals of the load power switch 3. When the hydraulic lift system reaches some predefined mechanical position, the limit switch 6 opens, in turn opening the load power switch 3 and stopping the hydraulic pump 4. Alternatively, the manual switch 5 can be opened which also opens the load power switch 3 and stops the hydraulic pump 4.
Every electrical apparatus has failure modes. Typically, the useful life of a power contactor is a function of its electrical contacts and the most common failure mode is a stuck or welded contact. If the contacts of the load power switch 3 were to stick or weld in a closed state, there is no way to release electrical power to the hydraulic pump 4 unless the circuit breaker 2 opens from excessive current. To ensure reliable operation of the hydraulic system, there is a need for a safety device to release power to the hydraulic pump 4 in the event of such a stuck or welded contact under all operating conditions. The battery supplies power to the hydraulic lift system at different levels under differing operating conditions. When only the internal electronics, floodlights and accessories 7 are on but current is not flowing through hydraulic pump 4, current requirements can range between slightly more than 0 amps and 10 amps, for example. When the hydraulic pump 4 is energized, this current can be in the range of 40 to 100 amps, for example.
Consequently, when only the internal electronics or floodlights 7 are operating, a current that exceeds the maximum current expected during normal operation is considerably less than the normal or maximum current expected when the hydraulic pump 4 is operating. As a result, the circuit breaker 2 must be selected to interrupt a current that is greater than the maximum expected under normal operating conditions when both the internal electronics and flood lights 7 are on and the hydraulic pump 4 is also being operated. Otherwise, the circuit breaker 2 would open under a normal operating condition. Therefore, a fault in only the circuit of the internal electronics and flood lights 7 that could cause damage or injury may not cause a current that is large enough to open the circuit breaker 2 when the hydraulic pump 4 is not operating.
More importantly, if the contacts of the load power switch 3 were to stick or weld in a closed state and there is no other fault in the circuit, the total current would not exceed the expected maximum operating current and therefore will not cause the circuit breaker 2 to open. In that event, the hydraulic pump 4 would continue moving the lift even if the operator opened the manual switch 5. Of course under this condition the lift may continue to operate until the lift comes to the end of its normal movement at which time the hydraulic pump will be overloaded causing an excessive current that would open the circuit breaker 2. However, by then, an occupant of a lift may be injured and additionally, the hydraulic pump 4 and/or other parts of the apparatus may be damaged.
Therefore, it is an object and feature of the invention to provide a circuit that can recognize when the maximum expected current through the power switching terminals of the load power switch 3 is exceeded under each of these operating conditions, indicating the existence of a failure mode, and deenergize the circuit in response to such a condition.